Breathtaking Saturn

Breathtaking Saturn

On Dec. 17th, Earth and Saturn will have their closest
encounter in nearly 30 years.

December 13, 2002: Thirty years
ago, Earth and Saturn had an extraordinary close encounter. The
ringed planet was only 1.2 billion km from Earth--about as close
as it can get--and its rings were tipped toward us. The view
through a telescope was simply breathtaking.

Next week it's going to happen again.

"On Dec. 17th, Saturn and Earth will be unusually close
together--about the same as thirty years ago," says NASA
astronomer Mitzi Adams. And once again the planet's rings are
tilted in our direction for maximum effect. "So get out
your telescope." says Adams. "Even a small one will
do."

Above: Photographer Ed Grafton captured this image of
Saturn from Houston, Texas, on Dec. 11, 2002. He used a 14-inch
telescope and a CCD camera. [more]

Observers very often gasp when they first see Saturn
through a telescope. Adams recalls her own first look: "It
was Nov. 1969, and I was a volunteer at the Fernbank Science
Center in Atlanta. We were tracking the Apollo 12 spacecraft
for an NBC news crew using the center's 36 inch telescope. It
was a very cold night, about 9o F. For fun
we rolled out a 10-inch telescope, too, and pointed it at Saturn.
The sight of the planet and its rings took my breath away. Or
maybe that was the cold!" she laughs. "Seriously, I
don't think I'll ever forget that image. Now, whenever I show
Saturn to 'first-timers,' I really enjoy watching their reactions,
remembering my own experience."

Dec. 17, 2002, is special because that's when Saturn and the
sun are on opposite sides of the sky. Astronomers call this "opposition."
When the sun sets, Saturn rises and it's up all night. Saturn
at opposition is close to Earth (see the diagram below) and therefore
bright.

Below: Saturn is "at opposition" when it and
the sun are on opposite sides of Earth. The size of Earth's orbit
is exaggerated for clarity. Saturn is 9 1/2 times farther from
the sun than Earth.

Oppositions of Saturn come every 13 months
or so. This one is the best in nearly 30 years because Saturn
is also near perihelion--its closest approach to the sun. Adams
explains: "Saturn's 30-year orbit is not a perfect circle.
It has the shape of an ellipse with one side 6% closer to the
sun than the other. When Saturn is closer to the sun it's also
closer to Earth ... and we get a great view."

Finding Saturn is easy, notes Adams. Look east after sunset.
Saturn will be there, rising, among the bright stars of the constellation
Taurus. Saturn is yellow in hue and doesn't twinkle like a star.
Shining at visual
magnitude -0.5, it's one of the brightest objects in the
winter evening sky. At midnight, adds Adams, Saturn will be almost
directly overhead.

On Dec. 18th, says Adams, you can use the Moon to find Saturn
because the pair will be close together. Glare from the full
Moon will impair your night vision, but not nearly enough to
wipe out Saturn.

Saturn is so bright, she explains, in part because its vast
rings are tipped toward us. They reflect sunlight very well.
The rings are 274,000 km wide--twice as wide as the planet Jupiter.
They are only a few tens of meters thick, however, which is why
they vanish when seen edge-on.

Such was the situation 7 years ago: the rings were edge-on
and practically invisible. This happens because Saturn is tilted
27o with respect to its own orbit, so the rings appear
to wobble as the planet goes around the Sun. Sometimes, like
now, they are easy to see, but not always.

Below: The eastern sky at 9:30 p.m. local time on Dec.
17 and 18, 2002, as seen from mid-northern latitudes. The blue
circles denote the changing position of the Moon. Southern hemisphere
sky watchers should invert this map and look northeast. [larger sky map]

Galileo himself was vexed by Saturn's wobble. He discovered
Saturn's rings in 1610. Altissimum planetam tergeminum observavi,
he wrote in Latin. "I have observed the highest planet triple:
o0o." That was how the rings looked through his primitive
telescope. Galileo's attempts to understand what he saw were
frustrated when, two years later, the blobs vanished. The rings
had tipped edge-on, but didn't know that. The coming and going
of Saturn's rings puzzled researchers for many years thereafter,
and indeed it wasn't until 1656 that Dutch astronomer Christiaan
Huygens figured out the correct explanation.

Astronomers have been observing Saturn's rings ever since--nearly
400 years. Even so, there's much we don't know about them. The
rings are made of icy chunks ranging in size from dust to large
houses. It looks like debris, but from what? A shattered moon?
A passing asteroid torn apart by Saturn's gravity? No one knows.
There is mounting evidence that the rings are much younger than
Saturn itself. Some researchers believe the rings appeared only
a few hundred million years ago--a time when the earliest dinosaurs
roamed our planet. (For more information about this intriguing
possibility, read the Science@NASA story "The
Real Lord of the Rings.")

A spacecraft called Cassini--a joint mission of NASA, the
European Space Agency and the Italian Space Agency--is en route
to Saturn now. It will arrive in July 2004 and become the first
spacecraft to orbit rather than fly by Saturn. (Six months later
Cassini will release its piggybacked Huygens probe for descent
through the thick atmosphere of Saturn's moon Titan.) Bristling
with scientific instruments and with plenty of time to use them,
Cassini may tell us a great deal about the ringed planet.

Meanwhile, the only instruments you need to enjoy Saturn are
your eyes and a small telescope. "Saturn and Earth will
be close together for many weeks," notes Adams. If you miss
it on Dec. 17th, she says, don't worry. The planet can take your
breath away for some time to come.

The Science and Technology Directorate at NASA's
Marshall Space Flight Center sponsors the Science@NASA web sites.
The mission of Science@NASA is to help the public understand
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more information

The
Real Lord of the Rings
(Science@NASA) Four hundred years after they were discovered,
Saturn's breath-taking rings remain a mystery.